Study of Hydrodynamics and Heat Transfer Induced by Multiple Droplet Train Impingements

Recently, spray cooling heat transfer has received considerable attention due to its ability to dissipate high thermal loads. However, the physical mechanisms of spray cooling remain not well-understood due to the complexity of sprays. In order to better understand the underlying physical mechanisms found in spray cooling, a number of studies have been conducted by isolating spray parameters and focusing on well-controlled droplet train impingement cooling schemes. Most of the previous droplet train impingement studies have mainly focused on the thermal performance of single, double and collinearly arranged triple droplet train impingement. In the current study, the heat transfer and hydrodynamic characteristics of various droplet train impingement arrays have been investigated experimentally. A piezo-electric droplet generation system has been designed and constructed, which is capable of producing well-controlled droplet trains arranged in various patterns (single, double, triangulated and hexagonal-arranged droplet trains). A translucent sapphire substrate, which was coated with a thin film ITO (Indium Tin Oxide), has been used as a heater in experiments. Well-calibrated high speed optical camera and IR thermal camera have been used to characterize the hydrodynamics and heat transfer of droplet train impingement. Droplet-induced crown propagation dynamics has been analyzed experimentally. A revised crown propagation model was proposed in the current study, which is capable of predicting the crown base diameter as a function of time. A transition from crown spreading to splashing has been observed by increasing droplet Weber number. Heat transfer measurements show that strong splashing was unfavorable for heat transfer due to the instability of the liquid film. For multiple droplet train impingement, it was found that impact spacing and impingement pattern play significant roles in terms of heat transfer performance. Furthermore, empirical heat transfer correlations have been postulated and fitted using experimental data for various impingement patterns. Results indicate that the postulated correlations are in good agreement with experimental data. Comparisons have been made between droplet train impingement and circular jet impingement for various impingement patterns. It has been found that droplet train impingement leads to much better heat transfer performance than circular jet impingement due to the effective mixing of fluid during droplet impingement process. In summary, the effects of droplet Weber number, impact spacing and impingement pattern on heat transfer and hydrodynamics during droplet train impingement have been explored and elucidated